Efforts to reduce manufacturing costs of chemical products are on-going with particular attention directed to reducing energy costs by implementing heat integration design, that is, the process-wide pairing of heat-releasing streams with heat-accepting streams. In addition, energy usage has been lowered by the adoption of more thermodynamically efficient design of unit processes such as distillation, heat exchange, and the like.
Distillation, although energy intensive, is a very common in the separation technique chemical and petrochemical industries. Distillation processes consume on the order of 95 percent of the energy used for all separation processes. Heretofore, most distillation designs have not been very good at determining thermodynamic efficiency due to past difficulty in evaluating the internal dynamics.
As is well known, a reversible process is most efficient from a thermodynamic perspective. Insofar as a process deviates from the reversible ideal, more energy is required than the theoretical minimum. The difference between the actual work (energy) which a process requires and its reversible work is defined as lost work. By analyzing a process for irreversible elements, it is possible to determine improvements which can increase efficiency and reduce energy consumption. A general background discussion of some thermodynamic concepts as applied to distillation columns is presented in Ognisty, "Thermodynamic Analysis of Distillation Columns," Paper No. 21a, 1993 A.I.Ch.E. Spring National Meeting, Mar. 30, 1993, incorporated herein by reference.
Kaibel, G. et al., Gas Separation and Processing, Vol. 4, No. 2, June 1990, pp. 109-114; and Kaibel, G., International Chemical Engineering, Vol. 32, No. 4, October 1992, pp. 631-641; describe analysis of distillation systems to improve thermodynamic efficiency for reduction of energy usage.
In a distillation process, heat is converted into work, namely, the separation of one or more chemical components from a mixture thereof. Areas of lost work include fluid flow momentum losses (pressure drop) and the presence of driving forces which are at great disequilibrium. Examples of these driving forces include heat transfer between fluids having different temperatures, such as mixing fluids at different temperatures, and mass transfer between streams which are not at equilibrium, such as mixing streams having different component concentrations. Therefore, a key to greater distillation efficiency is to stay as close to equilibrium conditions as is economically feasible by keeping driving forces small and minimizing pressure drop.
In a practical sense, areas of excessive driving forces (disequilibrium) in the column can result in lost work, defined as that portion of the total work which is necessary to overcome thermodynamic inefficiency. It would, therefore, be beneficial to enhance the thermodynamic efficiency of a methanol distillation unit, and where significant amounts of heat can be added or removed at temperature levels significantly different than the available steam, cooling water and other utilities, to implement heat integration in the column to reduce the amount of utilities required for the distillation. Process heat integration would use heat efficiently and reduce the amount of utilities the plant is required to generate for operation. Reducing steam consumption would, in turn, minimize the amount of oil, natural gas or coal used to generate steam. Reducing cooling water rates would similarly minimize makeup water rates and cleanup requirements. The amount of chemical and biological treatment of boiler feed water is directly related to total steam demand. Not only would efficient energy use reduce the costs of operating a distillation column, but would also minimize the waste involved with producing and maintaining utilities.